THE ART OF RECORDING
Part 4. AUDIO TRANSFORMER DESIGN
In all cases, the transformers were first wired with miniature MOGAMI shielded cable to NEUTRIK XLR connectors so they could be bench tested and measured.
The bench tests used square waves at 10Hz, 1kHz and 10kHz fed via a 50 Ohms output impedance TEKTRONIX CFG250 into the transformer's "primary" and the input/output monitored on a TEKTRONIX 465B Scope. No "loads" were used.
Measurements were made using a MCP BR2822 LCR Meter and checked on a MARCONI Universal Bridge TF2700.
The listening/recording tests were done using a METAXAS DVD Player connected [Van Den Hul Mic Hybrid cable] to a modified METAXAS AMI48 TRIBUTE Prototype Mixer . The modification was that the transformer signal path was detoured to the mixer side panel and terminated with Neutrik XLR panel plugs/sockets. This allowed us to insert a pair of the "Transformers Under Test" into the signal path quite easily.
The Mixer output was connected via Van Den Hul Mic Hybrid cable to our METAXAS WOLF ADC converter. The ADC SPDIF output was fed into the BNC SPDIF input of a Sound Devices 744T and recorded as a 24bit/96 wav. The resulting recordings were encoded as a 24b/96k AUDIO DVD using Minnetonka Software and the DVD was dubbed to a TASCAM 20bit CD4U burner to convert the 24b/96k files to 16b/44.1k.
Here is a VIDEO which goes over the bench tests of the transformers [11min 18Mb]:
Hi-Rez version of the same video [106Mb]:
All these samples are 16bit/44.1k
MARTIN BREEZE [Jazz-Male Vocals] ~ 50Mb
HANDEL'S MESSIAH [Female Vocals/Orchestra] ~ 25Mb
NOTE: These tracks start with Pink Noise to set the level.
1. Metaxas No.5MU
2. Lundahl 1538XL
3. Jensen JT-16B
4. UTC A11
5. Metaxas No.5RM
6. Metaxas No.5MU [repeat]
6. Jensen JT-10KB-DPC "reversed"
You can view a track from this concert to see how the mics/musicians were lined up. I was using Neumann 2xTLM50s in AB and 2xTLM50s in XY:
Listen to the huge "weight" in the sound and the "dimensionality" or 3D realism of the instruments. To me this is the key. And this is the problem with many of the commercial transformers I've tried. In general, they knock out the bottom octave [or "bloom"] and flatten the soundstage. Music through them sounds artificial and dry.
|Metaxas No.5MU [3/8" mu-metal core]|
|T1 Primary [input]||44.3H||4.57H||-350mH||34.5k||122.8k||22k||219.8pF||414pF|
|T2 Primary [input]||40H||2.2H||-319mH||33.4k||98k||20.2k||473pF|
|T1 Secondary [Output]||275.5H||29.5H||-2.2H||215k||764k||139k||432pF|
|T2 Secondary [Output]||234H||11H||-1.99H||200.2k||507k||126.4k||530pF|
|T1 Primary [input]||17.1H||698mH||-106mH||14.15k||43.8k||6.7k||324.5pF||679pF|
|T2 Primary [input]||15.95H||1082mH||-114.9mH||12.72k||38.9k||7.26k||344pF||697pF|
|T1 Secondary [Output]||437H||19.3H||-2.7H||338k||1.13M||172.9k||589pF|
|T2 Secondary [Output]||403.7H||27.6H||-2.9H||305.6k||1M||185k||630pF|
|T1 Primary [input]||25.28H||4.53H||-289mH||19.4k||69.7k||18.3k||266pF||603pF|
|T2 Primary [input]||22.06H||4.67H||-291mH||16.9k||60.2k||18.4k||586pF|
|T1 Secondary [Output]||101.1H||18.06H||-1148mH||78k||279k||72k||238pF||420pF|
|T2 Secondary [Output]||87.26H||18.7H||-1163mH||65k||240k||73.5k||404pF|
|T1 Primary [input]||5H||-1251mH||-13.3mH||3.6k||12.9k||841||74pF||36pF|
|T2 Primary [input]|
|T1 Secondary [Output]||1187H||-300H||-3.9H||814k||3.3M||245k||31.2pF|
|T2 Secondary [Output]|
|Metaxas No.5RM [1/4" radiometal core]|
|T1 Primary [input]||12.9H||3.7H||-720mH||10.3k||31k||50.7k||136pF||282pF|
|T2 Primary [input]||12.17H||3.54H||-709mH||9.83k||29.7k||50.4k|
|T1 Secondary [Output]||39.7H||14.6H||-2.94H||28.9k||124k||202k|
|T2 Secondary [Output]||37.86H||14.1H||-2.84H||27.5k||119.3k||202.6k|
|Jensen JT-10KB-DPC "reversed"|
|T1 Primary [input]||39.56H||-3.4H||-173mH||32.2k||103.6k||10.9k||70.1pF||120pF|
|T2 Primary [input]|
|T1 Secondary [Output]||627H||-50.1H||-2.8H||504k||1.66M||176.4k||131pF|
|T2 Secondary [Output]|
|OTHER MIC TRANSFORMERS|
|Lundahl 1550 "Amorphous Core"|
|T1 Primary [input]||8.3H||3H||-273mH||6.3k||18.6k||17.2k||230pF||485pF|
|T1 Secondary [Output]||59.8H||46.2H||-4.3H||38.4k||292k||269k||484pF|
|BEYER TR/BV 352 003 955|
|T1 Primary [input]||2.88H||553.4mH||91mH||2.37k||5.89k||17.13k|
|T1 Secondary [Output]||26.96H||5.2H||818mH||21k||54k||161k|
After recording over 200 concerts over the past 7 years, I wasn't prepared for the very big improvements in sound quality that could be afforded by winding my own transformers. I assumed, like most engineers, that the transformer winding art was sorted by the Jensens & Lundahls of this world. Fortunately it isn't. I'm looking forward to more improvements!
Since the WAVs are 44.1k, I suggest you burn them onto a CD for critical listening. Depending on the quality of your playback system, you will find the differences subtle or revelatory. If your system is up to it, the No.5MU transformer will be simply be MUCH MORE open, deeper, delicate, 3-D, expressive and incredibly musical than the others. You might also be seduced by some of the UTC A11's "magic".
Whilst researching the science of transformer design, I found that most important writings were actually done in the 30's, 40's and 50's with major contributions by engineer/journalist Norman Crowhurst. I've also been very privileged to know and work with local studio maverick Graham Thirkell. Graham has produced everything from tape recorders to his current practise of Studio Acoustic Design. Graham also worked with recording legend - Robert Fine [Mercury Recordings] when in Melbourne setting up Bill Armstrong's AAV studios. Graham's advice has been extremely valuable.
I've included the UTC A11's in the comparison because they are such a "glamorous" sounding transformer. From their measurements, you would normally conclude otherwise. Also, I've measured the ubiquitous BEYER transformer which I removed from my Stellavox tape recorders and mixers as it has been the main brand found in Studers and many German or Swiss machines. Sonically it isn't a match for the others included in the listening tests.
Textbooks will tell you that a good transformer is one which balances all the conflicting factors of impedance, inductance, leakage inductance and capacitance. But it's only when you start varying the windings ratios, wiring sizes, core materials, bobbin sizes, insulation and interleaving materials used, that you realise how important all these factors are to the final sonic outcome.
A good example is the sonic difference between our No.5 MU and No.5 RM. In essence, these transformers share the same windings/interleavings. The only difference is the 3/8" MU features English Mu Metal and the 1/4" RM features US Radio Metal. Yet they sound so different.
The next step will be to wind our first "pure silver" wire transformer. To keep our new Mixer "future-proof", the transformers and gain stages will be modular to allow for regular upgrades.
- Kostas Metaxas [copyright] metaxas at netspace.net.au